Although the cellular and molecular workings of the circadian pacemaker located in the suprachiasmatic nucleus (SCN) are being elucidated in striking detail, the neural mechanisms underlying diurnality versus nocturnality remain poorly understood. Current vertebrate research relies overwhelmingly on nocturnal rodent models; yet, diurnal species, including humans, present drastically different circadian rhythms. Recent work has revealed an endogenous rhythm in Fos-immunoreactivity in the lower subparaventricular, zone (LSPV), a principal target of the SCN that is reversed in a diurnal rodent species, the unstriped Nile grass rat, relative to that seen in lab rats. The research proposed here will test the hypothesis that the LSPV acts to 'switch' the coupling between the pacemaker and circadian rhythms in physiology and behavior in grass rats.